1. Field of the Invention
The invention relates to a method of dehalogenating hydrocarbon oils. The oils employed may be, for example, spent lubricating oils, which as a result of the treatment can be reused to produce re-refined products in a manner which conserves resources and does not pollute the environment. The oils may also be aromatic heat transfer oils, with which the method eliminates the corrosiveness caused by organochlorine compounds.
2. Discussion of the Background
During lubrication, lubricating oils experience increases in concentration of metal wear particles, as well as of their decomposition products, oxidation products, the fuel components. However, spent lubricating oils should not be regarded as waste products, because by means of various operations such as filtration, distillation, extraction with concentrated sulfuric acid or oleum, treatment with fuller's earth, and addition of new additives, they can be converted to highly valuable lubricants. These procedures regenerate a basic oil which poses no problems from a lubrication standpoint, using the spent lubricating oils as a starting material. However, certain impurities are carried along which are considered environmentally objectionable. Among these impurities are certain polychlorinated aromatics, which are mixed into the spent lubricating oils, sometimes by carelessness but sometimes intentionally or by technical accident (e.g., failure of a seal). Particularly significant impurities are the polychlorinated biphenyls (PCBs), which are widely disseminated as hydraulic oils in mining, and as insulating oils and dielectric media for transformers and capacitors, due to their incombustibility. For a number of years, PCBs have been a problem which has caused considerable apprehension, because they bioconcentrate similarly to DDT, and also when incinerated at temperatures below 1000.degree. C. they can give rise to much more hazardous substances, namely polychlorinated dibenzodioxins and polychlorinated dibenzofurans. Indeed, there is concern that such conditions may sometimes occur in internal combustion engines. This has led to the conclusion that PCB-contaminated used oils should not be reprocessed but should only be incinerated at high temperature, unless they can be dechlorinated at reasonable expense. Therefore, for reasons of environmental protection and conservation of raw materials, it is a problem of urgent interest to develop economical methods of reprocessing used oils so that they can be reused.
An obvious approach to eliminating halogenated compounds is to heat the oil with dispersed alkali metals. This technique is described, for example, in British Pat. Nos. 2,063,908 and 2,081,298, and U.S. Pat. No. 4,465,490. The results are satisfactory, but substantial excess metal is required, due to the fact that the reaction takes place only at the metal-oil interface. The subsequent removal of the metal is beset with safety problems.
Another method, described in U.S. Pat. Nos. 4,284,516, 4,326,090, and 4,447,667, employs naphthalene-sodium as a dechlorinating agent in homogeneous solution. This necessarily introduces an additional solvent, which is usually tetrahydrofuran. The reaction is rapid, even at low temperatures. However, it is uneconomical on an industrial scale, because of the high consumption of solvent. In this instance, also, as with dispersed alkali metals, a substantial excess of the dechlorinating agent is required. The naphthalene itself, used in the reagent, is an environmental hazard in that it is an aromatic hydrocarbon.
Methods of dechlorinating by hydrogenation are also known. They require a large capital investment because of the pressure apparatus required. Accordingly, they can be made profitable only at relatively high capacities.
Methods of dechlorinating organochlorine compounds by means of chemical reducing agents have also been described. Thus, dechlorination can be accomplished by means of nickel chloride and sodium boranate in isopropanol (W. H. Dennis, Jr., et a., 1979, Bull. Envir. Contam. Toxicol., 22 (6): 750-753), or by means of nickel chloride, triarylphosphine, and zinc dust, in dimethylformamide (U.S. Pat. No. 4,400,566), or by means of hydrazine hydrate in the presence of a palladium catalyst over activated carbon (Japanese No. 74-61143). The nature of the auxiliary reagents would be sufficient to disqualify these methods for industrial application, because of the novel waste disposal problems presented by the residues which are produced. Apart from this consideration, the auxiliary reagents are very costly.
With methods in which a mixture of alkali hydroxide and polyethylene glycol are reacted with the contaminated oil in the absence (U.S. Pat. No. 4,351,718) or in the presence (German OS No. 30 33 170, U.S. Pat. No. 4,400,552, U.S. Pat. No. 4,337,368, and European Pat. No. 0 118 858) of oxidizing agents, large amounts of reagents are also consumed.
In European Pat. No. 0 021 294, the dechlorination of toxic organic aromatic chlorine compounds is described, aimed primarily at treating manufacturing residues from 2,4,5-trichlorophenol production. The products are heated with alkali alcoholates (MOR) of monohydric alcohols with 1-5 carbon atoms, or polyalkylene glycols with 4-20 carbon atoms, or polyols with 2-5 carbon atoms and 2-3 hydroxyl groups, or monoalkyl ethers derived from the latter polyols and alcohols with 1-4 carbon atoms, employed in the presence of 0.5-1 equivalent of the free alcohol (based on the organically bonded halogen). Alternatively, mixtures of the aforementioned alcohols with alkali hydroxides and/or alkali carbonates may be used. In general, sodium glycolate/ethylene glycol is used for dechlorination. In order to use sodium ethoxide/methanol, one must resort to high pressures.
The method of European Pat. No. 0 021 294 completely decomposes the manufacturing residues. Based on what has been disclosed of the reaction conditions, it is not suitable for processing large amounts of only slightly contaminated oils which are intended for reuse. In addition, it involves heterogeneous reactions, and is therefore cumbersome to integrate into a continuous production process.